Since June, 1981, over 700 cases of AIDS have been reported to the Centers for Disease Control. Most cases have involved homosexual or bisexual men; the remainder involved heterosexual men and women, Haitians, and intravenous drug abusers. The etiology of the disease is probably a variously transmitted biological agent. Patients with AIDS have evidence of cellular immunodeficiency with cutaneous anergy, reduced numbers and altered proportions of T-lymphocytes, and impaired lymphocyte transformation. Opportunistic infections and Kaposi's sarcoma occurring in these patients are likely due to acquired immunodeficiency analogous to treatment with immunosuppressive drugs. A genetic deficiency of adenosine deaminase (ADA) has been causally associated with severe combined immunodeficiency disease (SCIDS). Patients with ADA deficiency are subject to recurring, chronic varied infections to which they often succumb. Many have no detectable lymphocytes in peripheral blood or bone marrow. Cell mediated immunity is severely impaired. T-cells defined by E rosettes are often absent and lymphocyte proliferative responses are severely limited. Given the close clinical resemblance between AIDS and SCIDS, we propose to measure ADA levels in erythrocytes of 20 to 30 AIDS patients in order to determine whether a clinical manifestation of the disease process is associated with a deficiency of ADA. We propose, during Phase I, to determine whether or not there is an ADA deficiency in circulating erythrocytes of AIDS patients. If so, we plan to expand the study in Phase II and also develop a convenient assay for adenosine deaminase as part of a rapid screening test for AIDS.